Kobe Steel, Ltd.

Muneaki Ikeda

Toshio Murakami

Masaaki Miura

Katsura Kajihara

Michiharu Nakaya

Tomokazu Masuda

Yoichi Mukai

Yukihiro Utsumi

Fumio Yuse

Junichiro Kinugasa

Kouji Kasuya

Hideo Hata

Kenji Saito

Hiroshi Akamizu

Koichi Sugimoto

Tetsuji Hoshika

Shinji Kozuma

Tadao Murata

Kosuke Shibata

The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is −1.2 to −0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 μm.

High-strength steel sheet and method for producing same

In one aspect of the present invention, a hot-dip galvannealed steel sheet includes a steel sheet and a hot-dip galvannealed layer on the surface of the steel sheet. The steel sheet has a predetermined composition and has an average oxygen concentration of 0.10 mass % or less in the region of 1 &#x3bc;m from the interface between the steel sheet and the hot-dip galvannealed layer toward the steel sheet. The metal microstructure of the steel sheet at a position of t/4 where t represents the sheet thickness of the hot-dip galvannealed steel sheet includes 50 to 85 area % of martensite, 15 to 50 area % of bainite, and 5 area % or less of ferrite.

Alloyed hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet production method

The present invention relates to a high-strength steel sheet which satisfies a specific chemical component composition, and which is configured such that: when the metal structure thereof is observed with a scanning electron microscope, polygonal ferrite is 30 to 60% by area; when the metal structure thereof is observed with an optical microscope, MA mixed structure is 21% by area or less; when the metal structure thereof is measured by an X-ray diffraction method, retained austenite is 8% by volume or more; an area ratio of polygonal ferrite having a circle equivalent diameter of 35.0 &#x3bc;m or more is 0% and an area ratio of polygonal ferrite having a circle equivalent diameter of 10.0 &#x3bc;m or more and less than 35.0 &#x3bc;m is 10% or less in the polygonal ferrite; and connectivity as defined by a specific relational expression is 10% or more.

High-strength steel sheet, high-strength galvanized steel sheet, method for producing high-strength steel sheet, and method for producing high-strength galvanized steel sheet

Disclosed herein is a high-strength plated steel sheet having a plated layer on the surface of a base steel sheet and containing predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and a hard layer consisting of a structure which is mainly composed of martensite and bainite and in which the average grain size of prior austenite is 20 &#x3bc;m or less. The average depth D of the soft layer is 20 &#x3bc;m or greater, and the average depth d of an internal oxide layer is 4 &#x3bc;m or greater and smaller than D.

High-strength plated steel sheet having excellent plating properties, workability, and delayed fracture resistance, and method for producing same

The present disclosure relates to a high-strength hot-dip galvannealed steel sheet excellent in bake hardenability and bendability and having a component composition containing, in mass %, C: from 0.05 to 0.30%, Si: from 0.5 to 3.0%, Mn: from 0.2 to 3.0%, P: from 0 to 0.10%, S: from 0 to 0.010%, N: form 0 to 0.010%, and Al: from 0.001 to 0.10%, with the remainder being iron and unavoidable impurities. The steel sheet has a steel structure containing, in terms of area percentage, martensite: form 50 to 85% and ferrite: 0% or more and less than 5%, with the remainder being bainite. The steel sheet has a dislocation density of 5.0&#xd7;10mor more, a solute carbon amount of 0.08 mass % or more and a tensile strength of 1180 MPa or more.

High-strength hot-dip galvannealed steel sheet having excellent bake hardening property and bendability

Disclosed is an ultra high strength steel plate with at least 1100 MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the aforementioned soft phases is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1.0. The ultra high strength steel plate has at least 1100 MPa of tensile strength.

Ultrahigh-strength steel sheet with excellent workability, and manufacturing method thereof

In a steel sheet having a specific chemical composition and having a microstructure including ferrite that is a soft first phase by 20-50% in terms of the area ratio, the remainder being tempered martensite and/or tempered bainite that is a hard second phase, the microstructure of steel of a surface layer section of the steel sheet from the surface to the depth of 100 &#x3bc;m and a center section of t/4-3t/4 (t is the sheet thickness) is controlled.

High strength cold-rolled steel sheet and manufacturing method therefor

In a high strength cold-rolled steel plate having a specific chemical composition, a soft first phase (ferrite) has an area ratio of 20-50%, the remainder being a hard second phase (tempered martensite and/or tempered bainite), among all the ferrite grains, ferrite grains that have an average grain diameter of 10-25 &#x3bc;m account for a total area ratio of 80% or more, the number of the cementite grains that have an equivalent circle diameter of 0.3 &#x3bc;m or more is more than 0.15 piece and 1.0 piece or less per 1 &#x3bc;mof ferrite, and the tensile strength is 980 MPa or more.

High strength cold-rolled steel sheet exhibiting little variation in strength and ductility, and manufacturing method for same

A high-strength cold-rolled steel sheet has a chemical composition including C of 0.05% to 0.30%, Si of greater than 0% to 3.0%, Mn of 0.1% to 5.0%, P of greater than 0% to 0.1%, S of greater than 0% to 0.02%, Al of 0.01% to 1.0%, and N of greater than 0% to 0.01%, in mass percent, with the remainder including iron and inevitable impurities. The steel sheet has a microstructure containing ferrite as a soft primary phase in an area percentage of 20% to 50% with the remainder including tempered martensite and/or tempered bainite as a hard secondary phase. The ferrite grains are adapted to contain cementite particles having an appropriate size in an appropriate number density.

High-strength cold-rolled steel sheet having small variations in strength and ductility and manufacturing method for the same

The present invention is the thin steel sheet containing C, Si, Mn, P, S, Al, Mo, Ti, B, and N wherein a value Z calculated by the equation described below is 2.0-6.0, an area ratio against all the structure is 1% or above for retained austenite and 80% or above for total of bainitic ferrite and martensite, a mean axis ratio of the retained austenite crystal grain is 5 or above, and tensile strength is 980 MPa or above whereValue Z=9&#xd7;[C]+[Mn]+3&#xd7;[Mo]+490&#xd7;[B]+7&#xd7;[Mo]/{100&#xd7;([B]+0.001),andthe thin steel sheet has 980 MPa or above tensile strength and enhanced hydrogen embrittlement resistance properties.

High-strength thin steel sheet

The ultrahigh-strength steel sheet contains, in mass %, C: 0.05-0.25%, Si: 1.00-2.5%, Mn: 2.0-4.0%, P: 0.1% or below (not inclusive of 0%), S: 0.05% or below (not inclusive of 0%), Al: 0.01-0.15%, Ti: 0.003-0.10%, N: 0.01% or below (not inclusive of 0%), the balance comprising iron with inevitable impurities, and is a composite structure steel sheet comprising 10-50 area % ferrite and 50 area % or above martensite, in which the average circle-equivalent grain diameter of ferrite grains is 2.0 &#x3bc;m or below, the average aspect ratio of ferrite grains is 2.0 or below, and the tensile strength is 1,100 MPa or above. Accordingly, the ultrahigh-strength steel sheet is excellent in hydrogen embrittlement resistance and workability.

Ultrahigh-strength steel sheet excellent in hydrogen embrittlement resistance and workability, and manufacturing method therefor

The invention relates to an ultrahigh-strength thin steel sheet excellent in the hydrogen embrittlement resistance, the steel sheet including, by weight %, 0.10 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less of Al, 0.003 to 2.0% of Cr, and a balance including iron and inevitable impurities; in which grains of residual austenite have an average axis ratio (major axis/minor axis) of 5 or more, the grains of the residual austenite have an average minor axis length of 1 &#x3bc;m or less, and the grains of the residual austenite have a nearest-neighbor distance between the grains of 1 &#x3bc;m or less.

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Kakogawa, Japan

Muneaki Ikeda